1. Field of the Invention
This invention relates generally to impedance matching circuits. More particularly, it relates to the use of a digital filter such as a sigma delta filter to provide a programmably adjustable AC impedance matching circuit.
2. Background of Related Art
Impedance matching is important in many applications, most notably in matching the impedance of a telephone device such as a modem or a telephone line interface and telephone connected to a telephone line, with a central office at the other end of the telephone line.
To maximize the signal to noise ratio of customer premises equipment attached to a telephone line, e.g., the voltage level, it is desired that the impedance of the customer premises equipment match the impedance of the central office.
Impedance matching in this disclosure refers to image impedance matching. It is desirable to match impedances as required by the various countries. Moreover, it is also desirable to maximize return loss, e.g., to greater than 20 decibels (dB).
AC impedance includes reactive elements which cause the frequency components of the transmitted signal to vary. To maximize return loss, the AC termination impedance of a telephone line interface should be the same as the AC termination at the telephone company central office. This is commonly known as an image match. However, as evidenced by numerous standards throughout the world, AC terminations vary between telephone company central offices throughout the world.
To accommodate the numerous standards throughout the world, conventional telephone or modem devices switch in external components to match the impedance in the particular country. However, this technique requires the inclusion of components which likely may not be used when the equipment is installed in any particular country. These extra components increase costs, decrease overall reliability of the device, and cause difficulties in manufacture for worldwide marketplace.
There is a need to simplify the design of a telephone line interface for both a telephone and/or a modem device in a way that simplifies manufacturing concerns in a worldwide marketplace, and which reduces overall costs.
In accordance with the principles of the present invention, an AC impedance matched circuit comprises a digital filter including a transfer function based on a desired output AC impedance. A voltage controlled current source is controlled by an output of the digital filter. A reference impedance is in parallel with the voltage controlled current source.
A method of matching AC impedance in accordance with another aspect of the present invention comprises digitally filtering an output signal using a transfer function based on a desired AC impedance. A voltage controlled current source is controlled with the digitally filtered output. A voltage is developed across a reference impedance with the current output from the voltage controlled current source.